1. Field of the Invention
The present invention relates to a method for adjusting a nickel-metal hydride storage battery.
2. Related Art
In recent years, various types of nickel-metal hydride storage batteries have been used as power sources for portable devices or power sources for electric vehicles or hybrid electric vehicles. The nickel-metal hydride storage batteries are normally designed so that the capacity of a negative electrode is larger than the capacity of a positive electrode. Thus, the discharge capacity of the battery is limited by the positive electrode capacity (hereinafter, also referred to as “positive electrode regulation”). This positive electrode regulation makes it possible to suppress an increase in internal pressure of the battery during overcharge and overdischarge. From contrast between the negative electrode and the positive electrode, an excess uncharged portion available for charge is referred to as charge reserve, while an excess charged portion available for discharge is referred to as discharge reserve.
Meanwhile, it has been found from recent searches that, in some nickel-metal hydride storage batteries, a small amount of hydrogen gas continues to permeate through and leak out of a battery case. When hydrogen gas leaks to the outside in this way, hydrogen is released from the hydrogen absorbing alloy of the negative electrode according to a hydrogen leakage amount in order to keep balance of hydrogen partial pressure in the case. This decreases the discharge reserve of the negative electrode. Thus, hydrogen leakage advances very slowly and hence does not cause any problem in relatively short-term use.
However, in long-term use, the balance between the positive electrode capacity and the negative electrode capacity becomes lost and also the negative electrode capacity decreases, and the discharge reserve runs short. As a result, the nickel-metal hydride storage battery is subjected to the negative electrode regulation (meaning that the discharge capacity of the battery is limited by the negative electrode capacity). This results in a decrease in discharge capacity and a large deterioration in battery characteristics. When the nickel-metal hydride storage battery is to be used as a power source of an electric vehicle, hybrid electric vehicle, etc., a long-term service life is demanded. Thus, the above deterioration in battery characteristics would be problematic.
To solve the above problems, there is proposed a method for reproducing a nickel-metal hydride storage battery having a battery capacity decreased due to a decreased in discharge reserve (see Patent Document 1, for example). Patent Document 1 discloses a method including overcharging a nickel-metal hydride storage battery, releasing at least part of oxygen gas generated from a positive electrode to the outside of the battery, thereby increasing the discharge reserve capacity of a negative electrode.